Permeation of the placenta and diffusion membrane

2021-06-09 02:33 PM

During the first months of pregnancy, the placental membrane is still thick because it is not fully developed. Hence its permeability is low. Furthermore, the surface area is small because the placenta has not yet developed significantly. So, the total diffusivity is very small at first.

The main function of the placenta is to deliver and carry nutrients and oxygen from mother to baby, and to receive excretions back into the mother's body.

During the first months of pregnancy, the placental membrane is still thick because it is not fully developed. Hence its permeability is low. Furthermore, the surface area is small because the placenta has not yet developed significantly. So, the total diffusivity is very small at first.

In contrast, in the later stages of pregnancy, permeability increases because the membrane thins and the surface area is enlarged, thereby increasing diffusion across the placenta.

Sometimes, there is also a "break" in the placental membrane, causing foetal blood cells to enter the mother's body, but also maternal cells to enter the foetus. But it is also rare because the blood-placenta barrier is formed.

Diffusion of oxygen across the placental membrane

Like oxygen diffusion across the pleura (Chapter 40), dissolved oxygen dissolved in the blood of the maternal sinuses enters the foetal blood by simple diffusion, controlled by a pressure gradient of oxygen from the blood. mother to enter the foetal blood. Near the end of pregnancy, the oxygen partial pressure (PO2) of the maternal blood in the placental sinus is about 50 mmHg, and the PO2 in the foetal blood after oxygenation in the placenta is about 30 mmHg.

Figure. Oxygen-haemoglobin dissociation curve for maternal and foetal blood, showing that foetal blood can carry a greater amount of oxygen than maternal blood can give a given blood PO2.

Thus, the mean pressure gradient for the diffusion of oxygen across the placenta is about 20 mmHg.

People wonder how the foetal blood has only a partial pressure PO2 of 30 mmHg and yet provide adequate oxygen to the fetus. There are three reasons why even low levels of PO2 can still provide enough oxygen to the fetus.

First, foetal haemoglobin is Hb-F, a type of haemoglobin that is synthesized in the fetus before birth. The figure shows a comparison of the oxygen dissociation curves of maternal haemoglobin and foetal haemoglobin, showing that with low PO2, foetal haemoglobin also carries 20 to 50 percent oxygen from the mother.

Second, the foetal haemoglobin is usually larger than that of the mother, which is an important factor in increasing the amount of oxygen transported to the foetal tissues.

Third, the Bohr effect, (explained in chapter 41) provides a mechanism to enhance foetal blood oxygen transport. That is, haemoglobin can carry more oxygen at a low pCO2. The foetal blood entering the placenta contains a large amount of CO2, which then diffuses from the foetal blood into the mother's blood. Loss of CO2 makes the foetal blood more alkaline, receiving more CO2 makes the mother's blood more acidic.

These changes result in an increase in the oxygen-binding capacity of the foetus. In which, the amount of oxygen is still more than from the mother's blood while enhancing the oxygen absorption in the foetal blood.

Thus, the Bohr effect works in two different ways in the mother and the foetus. This effect is called the double Bohr effect.

With more oxygen intake across the placental membrane, despite the fact that the blood leaving the foetus only has a PO2 pressure of 30 mmHg.

The total diffusion capacity of the entire placenta is about 1.2 ml/min, facilitating lung respiration at birth.

Diffusion of CO2 across the placental membrane

CO2 is constantly being formed in the tissues of the foetus and will be released into the mother's blood through the placenta. The partial pressure pCO2 of foetal blood is 2 to 3 mm Hg, which is higher than that of maternal blood. This small pressure gradient is sufficient for CO2 diffusion because CO2 dissolves in placental blood 20 times faster than oxygen.  

Diffusion of nutrients across the placental membrane

Other substances diffuse into the foetal blood similarly to oxygen. For example, in the late stages of pregnancy, the foetus often uses more sugar and gets it from the mother's body. To provide this adequate supply, the trophoblast cells lining the placental villi facilitate the diffusion of glucose, which is transported by the trophoblasts in the cell membrane. Even so, the glucose concentration in foetal blood is only 20 to 30 percent of that in maternal blood.

Because fats are highly soluble in cell membranes, fatty acids also diffuse from maternal blood into foetal blood, but more slowly than glucose. In addition, some other substances such as ketones or potassium, Na, and Cl also diffuse relatively easily into the foetal blood.

Excretion of waste through cell membranes

Foetal excretory products also diffuse across the placental membrane into the maternal blood and are then excreted with maternal excretion products, similar to CO2 diffusion. These products include urea, uric acid, and creatinine.

The concentration of urea in the foetal blood is slightly higher than that in the maternal blood because urea diffuses easily across the placental membrane. In contrast, creatinine is more difficult to cross the placental membrane. Therefore, foetal excretion depends mainly on the diffusion gradient across the placental membrane and its permeability. The concentration of substances in the foetus is higher than in the mother's blood, so it should diffuse continuously from the foetal blood to the mother's blood.